Directed by Clint Eastwood, the film will tell the true story of former Navy SEAL, Chris Kyle, who holds the record for the greatest number of sniper kills in US military history.

Sadly, Kyle and a companion were shot and killed in February last year at a shooting range in Texas. A troubled Marine veteran is currently on trial for his murder.

Cooper, who's also producing the project, spoke to the soldier before he died.

"Thank God I got to talk to him once on the phone," Cooper told USA Today. "It was a very quick conversation. But I did tell him how serious I was about making this movie. And he should just know that whatever fears he had about Hollywood, to just put them aside and trust me. That I was going to do everything I could to tell this story.

"I had to do right by him and his family, there was really no choice. You're sitting across the dining-room table talking to this person's father and mother. And his children and wife are there. And he's passed away. Knowing that they are putting all of their stories in your hands and the responsibly of that, it's actually unique."

Cooper transformed himself for the role, putting on 40 pounds to look every inch the marine, and training with real-life with Navy SEALs. But despite all the preparation, he still found it shocking to look through a rifle scope for the first time.

"Even though it was actors through the scope, they were human beings. It was a whole other deal. I remember getting a chill through my body realising what he had to do."